Carcinomas, the most common human malignancy, arise from epithelial cells. Progression of epithelial cancers begins with the disruption of cell-cell contacts as well as the acquisition of a migratory (mesenchymal-like) phenotype. This phenomenon, which is called an epithelial-to-mesenchymal transition (EMT), is considered to be a crucial event in late stage tumor progression and metastasis.
The secreted protein TGF-β suppresses tumor growth initially largely due to its growth inhibitory action on tumor cells of epithelial origin, then at later stages promotes tumor cell progression and metastasis. One mechanism by which TGF-β can promote tumor progression is through the induction of an EMT.
Due to the dual role that TGF-β plays in carcinogenesis, direct inhibitors of TGF-β may be risky since, while they could benefit late stage tumors, they could also accelerate preneoplastic lesions. A better therapeutic may be one that inhibits the pro-oncogenic EMT-promoting action of TGF-β, while leaving the tumor suppressor growth-inhibitory action of TGF-β unaffected. To develop such an inhibitor it would be necessary to identify the point at which there is a bifurcation of the TGF-β signaling pathway such that the mediators in one branch of the pathway participate in the EMT response, but not the growth inhibitory response to TGF-β. Therapeutics that inhibit mediators that lie exclusively in the EMT-promoting branch of the TGF-β signaling pathway will reduce metastasis while having little or no effect on the acceleration of preneoplastic lesions.
No TGF-β signal pathway specific components have been generally identified that promote or mediate the EMT-promoting action of TGF-β, yet are not involved in the growth inhibitory action of TGF-β.
In contrast, an endogenous protein (the YY1 nuclear factor) has been identified that is able to interfere with (as opposed to promote) the protumorigenic EMT action of TGF-β, while leaving the tumor-suppressing action (growth inhibition) intact (Kurisaki et al., 2004).
Inhibitors that target TGF-β ligands, receptors and the Smad signaling proteins are known. Specifically, soluble receptor ectodomains, antibodies and other binding proteins are able to act as antagonists by interacting with TGF-β ligands and sequestering them away from cell surface receptors. Small molecules are available that inhibit the kinase activity of the Type I TGF-β receptor and endogenous inhibitors of the Smad signaling proteins are also known. Since all of these signaling pathway components are involved in both the pro- and anti-carcinogenic actions of TGF-β, these inhibitors that target them may benefit late stage tumors, however, they could also accelerate preneoplastic lesions.